1. Field of the Invention
The present invention relates to a method for operating a memory chip, and more particularly, to a method for dynamically adjusting the operation of a memory chip and an apparatus for measuring the thickness of an oxide-nitride-oxide (ONO) layer in the memory chip.
2. Description of the Related Art
Generally speaking, the operation window of the typical flash memory has two states: one is a program state, and the other one is an erase state. In order to increase the reliability of the memory, preferably, the program state should be separated from the erase state as much as possible such that a medium voltage can be defined to read these two states. However, as shown in FIG. 1, after many times of read-write operation, the operation window of the memory may be changed.
FIG. 1 schematically shows a relationship diagram of cycling times and threshold voltage (i.e. Vt), and a thickness of the tested ONO layer is about 200 Å. Referring to FIG. 1, wherein ▴ line represents a threshold voltage of a device after writing, ◯ line represents a threshold voltage of the device after losing the storage charge during writing state, ▪ line represents a threshold voltage of the device after erasing, and ▬ line represents a change of a threshold voltage after keeping the device in erasing state for a long time. After cycling operated (with a current of 10 μA) for a certain period of time, as the operating times increase, more charge loss (i.e. GL in the diagram) and more charge gain (i.e. CG in the diagram) are occurred. As a result, the window between the program voltage (i.e. PV in the diagram) and the erase voltage (i.e. EV in the diagram) is reduced, and problems may be occurred in the read word line voltage (i.e. RDWL in the diagram) between PV and EV after the cycling times exceed 10K.
In the prior art, RDWL was not adjusted after the product was manufactured. However, since there are some variances in the fabricating process, some methods for adjusting the size of RDWL based on the cycling times in the product operation have been disclosed in the field recently. Nevertheless, such methods only use a single memory cell (a single bit is used as a baseline) for measurement; if some unexpected problem is happened in the memory cell which is used as the baseline, the whole adjustment will be inaccurate.